1. Field of the Invention
The present invention relates to a method of forming a deposited film, and more particularly, to a silicon-based deposited film, and a method of forming a photovoltaic element using a silicon-based deposited film such as a solar cell.
2. Description of the Related Art
High frequency plasma CVD is an excellent method of mass-producing a silicon-based deposited film because a large area film can be formed easily at a low temperature and the process throughput is improved. A solar cell which is an application of a silicon-based deposited film to a product has an advantage in that the energy source is unlimited and the power generation process is clean as compared with a case of existing energy utilizing fossil fuel. However, in order to make a solar cell widely available, further reduction of costs is necessary. In attaining this, one important technical problem to be solved is to improve the film forming rate by high frequency plasma CVD and to establish technology for further improvement in the characteristics.
Japanese Patent Application Laid-Open No. H11-330520 discloses technology for a method of manufacturing a crystalline silicon-based deposited film layer. According to Japanese Patent Application Laid-Open No. H11-330520, a silicon-based deposited film layer can be formed at a high rate under the conditions under which a silane-based gas and hydrogen gas are included, the pressure inside a reaction chamber is set to be 666.61 Pa or more, and the distance between a substrate and an electrode is 1 cm or less. Further, a photoelectronic conversion device using the silicon-based deposited film layer has high conversion efficiency.
Japanese Patent Application Laid-Open No. 2000-252484 discloses technology of forming an amorphous silicon film under the conditions under which the partial pressure of an SiH4-based gas is 159.986 Pa or more and 2666.44 Pa or less, the interelectrode distance is 8 mm or more and 15 mm or less, and diluting hydrogen gas is four times or less of the SiH4-based gas.
Further, Japanese Patent Application Laid-Open No. H11-243219 discloses technology for a stacked photovoltaic element formed by stacking at least a constitution element with a pin junction element comprising a microcrystalline semiconductor in the i-type layer and a constitution element with a pin junction comprising an amorphous semiconductor in the i-type layer. According to Japanese Patent Application Laid-Open No. H11-243219, by providing a stacked photovoltaic element in which a current value is determined by the constitution element with the pin junction comprising the microcrystalline semiconductor in the i-type layer, photodegradation of the photovoltaic element can be suppressed to improve the characteristics.
The technologies disclosed in the above-mentioned patent documents and the like gradually improve the characteristics of a silicon-based deposited film formed by plasma CVD. For example, by carrying out plasma CVD at a relatively higher pressure (600 Pa or more) with a smaller interelectrode distance (10 mm or less) as compared with a conventional method, a deposited film containing microcrystalline silicon having relatively good characteristics can be formed at a high deposition rate of 1 nm/sec or more. A photovoltaic element such as a solar cell formed by those technologies has an improved conversion efficiency, a suppressed degradation ratio, and thus, better characteristics. Further, similarly, the characteristics of amorphous silicon are also improved by adjusting conditions for forming the deposited film.
However, in order to attain more excellent characteristics and further reduction of costs of the above-mentioned photovoltaic element or the like, there are still various problems to be solved.
As a first problem, in order to improve the conversion efficiency, it is necessary to find the well-balanced optimum conditions for forming a deposited film which is improved in both a short circuit current value and an open circuit voltage value at the same time.
The short circuit current value can be improved by, for example, making larger the crystallization ratio and the crystal grain diameter of the deposited film, and by making lower the defect density of the crystal boundaries. However, it has been found that a larger short circuit current value tends to make the open circuit voltage value smaller.
On the other hand, it has been found that, when the deposited film is amorphous (when the amount of amorphous component in the deposited film increases), the open circuit voltage value tends to become larger while the short circuit current value tends to become smaller.
In this way, the short circuit current value and the open circuit voltage value tend to be mutually contradictory depending on the crystallization ratio in the deposited film and other characteristics. Therefore, to find the well-balanced optimum conditions for forming the deposited film which is improved in both the short circuit current value and the open circuit voltage value at the same time, a lot of experiments are necessary, and adjustment of the conditions is extremely difficult.
As a second problem, in order to suppress the degradation, it is necessary to find the optimum conditions for forming the deposited film which can suppress the degradation ratio.
In a deposited film containing microcrystalline silicon, there is a problem that, when the crystal grain diameter is small, or when there are many defects in the crystal boundaries, it is impossible to attain an open circuit voltage value which is large enough for a solar cell, and the short circuit current value is small. In addition, there is also a problem that the film quality is degraded and the electrical characteristics and the like are lowered with the elapse of time.
Further, depending on the conditions for forming a deposited film, the ratio of microcrystals in the deposited film is decreased and the amount of the amorphous component is increased relatively. Depending on the thickness of the deposited film, in the case of light irradiation for a long period of time, weak bonds of those bonds constituting a network are broken and the number of dangling bonds increases, which has an adverse effect on the characteristics. When such a deposited film is adopted as an i-type layer of a pin junction of a photoelectric conversion device such as a solar cell, depending on the layer structure of the solar cell, degradation of the characteristics due to photodegradation is increased. Although those characteristics change depending on the conditions for forming the deposited film, the correlation between the conditions for forming the deposited film and the above-mentioned characteristics is not necessarily clarified, and thus, it is difficult to form a deposited film containing microcrystals having the optimum characteristics required for a photovoltaic element such as a solar cell.
On the other hand, the degradation ratio of amorphous silicon is not necessarily fixed, and significant differences exist in the degradation ratio depending on the amorphous structure. Therefore, it is desirable to form amorphous silicon having a low photodegradation ratio with a high open circuit voltage maintained which is a characteristic of amorphous silicon. However, it is generally difficult to form such amorphous silicon.
Further, the number of parameters to be controlled when a deposited film is formed in order to solve the first and second problems is large, and it is extremely difficult to determine the optimum range in which the characteristics are more excellent. More specifically, in the case of a method of forming a deposited film by plasma CVD, when, for example, in a solar cell having a structure in which two layers of a pin structure are stacked, that is, a so-called double structure, in order to attain conversion efficiency of about 10%, there is a certain extent of latitude in the combination of the respective parameters. Therefore, even if the conditions for forming the deposited film are not necessarily the optimum conditions, passable characteristics to function as an element can be obtained. However, when improvement of the conventional characteristics is attempted, it is extremely difficult to specify the conditions for forming the deposited film.
As a third problem, in order to improve independence from an apparatus, it is necessary to find a relationship which can be used when diverting the optimum conditions for forming the deposited film determined with an experimental apparatus for another apparatus with ease.
Conditions for forming a deposited film containing microcrystalline silicon or a deposited film substantially comprised of amorphous silicon include high frequency power, bias voltage, bias current, source gas flow rate, diluting gas flow rate, substrate temperature, pressure, and interelectrode distance. Because, generally, the conditions for forming a deposited film change much depending on the required characteristics of the deposited film, the structure of the deposited film forming apparatus, and the like, it is not easy to determine the optimum combination.
For example, in the case of a scale-up of the conditions for forming a deposited film determined with an experimental apparatus to those for a mass production apparatus, dimensions of the apparatus, such as the capacity of a space in which the deposited film is formed, the area of high frequency electrode, the position and the cross sectional area of an exhaust port, and the like are different. Therefore, if the conditions for forming the deposited film determined with the experimental apparatus are adopted as such for the mass production apparatus, although a certain extent of tendency or correlation can be grasped between the conditions for forming the deposited film and the characteristics of the deposited film, it is difficult to reproduce the characteristics as designed within a short period. More specifically, even if the conditions for forming a deposited film determined with the experimental apparatus are used as such, or after correction of the scale-up of the apparatus using a simple comparison expression, when a deposited film is actually formed with the mass production apparatus, the characteristics are not necessarily reproduced. As a result, it is necessary to determine again the optimum conditions using the mass production apparatus itself.